This revised application, submitted in response to Program Announcement 05-090 "Methodology and Measurement in the Behavioral Sciences," proposes an experimental analysis of behavioral assessment reactivity, while generating additional efficacy data on a promising HIV prevention intervention for South African sexually transmitted disease (STD) clinic patients. Our primary aim is to examine the public health impact and mechanisms underlying the effects of HIV risk behavior assessment on HIV risk behavior and on HIV prevention intervention outcomes in a sample of STD clinic patients in Cape Town, South Africa, an area severely affected by HIV and HIV-related stigma. 1500 patients (50%) women will be randomly assigned to condition in a extension of the Solomon Four Group experimental design. With an additional type of baseline assessment, the experiment is a 2 (HIV risk reduction intervention: enhanced vs. standard of care counseling) x 3 (baseline behavioral interview: none vs. general vs. detailed) factorial design. The two experimental baseline behavioral interviews are analogous to those currently used in major HIV prevention trials, and differ in the amount they require participants to focus on specific instances of risk behavior. We expect that the detailed interview will lead to more assessment reactivity than the general interview and that the detailed interview alone will lead to reductions in risk behavior. We will also test hypotheses based on variants of self-awareness theory regarding the mechanisms that mediate and moderate assessment reactivity. Results of this study will be relevant to the field of prevention science as well as to HIV prevention in South Africa. This study will be the first to experimentally disentangle the influence of baseline assessments from that of an HIV prevention intervention effective at promoting safer sexual behaviors. The study will also be the first in the health domain to explore the cognitive and interpersonal processes by which assessment reactivity occurs and the factors that moderate behavior change following self-reports of risk behavior. Regarding public health benefits, these results will help to determine how to maximize the effects of prevention interventions disseminated to community based organizations, and may help to develop brief, motivation-enhancing HIV risk-reduction interventions that capitalize on assessment reactivity. Results will also be relevant to microbicide trials that often use repeated detailed behavioral assessments, which may alter participants'willingness to use the microbicide and distort the outcomes.